Method to elimate silicide cracking for nand type flash memory devices by implanting a polish rate improver into the second polysilicon layer and polishing it

ABSTRACT

In one embodiment, the present invention relates to a method of forming a flash memory cell, involving the steps of forming a tunnel oxide on a substrate; forming a first polysilicon layer over the tunnel oxide; forming an insulating layer over the first polysilicon layer; forming a second polysilicon layer over the insulating layer by depositing an second polysilicon layer having a first thickness, and then using chemical mechanical polishing to form a second polysilicon layer having a second thickness, wherein the second thickness is at least about 25% less than the first thickness; forming a tungsten silicide layer over the second polysilicon layer by chemical vapor deposition using WF 6  and SiH 4 ; etching at least the first polysilicon layer, the second polysilicon layer, the insulating layer, and the tungsten silicide layer thereby defining at least one stacked gate structure; and forming a source region and a drain region in the substrate, thereby forming at least one memory cell.

TECHNICAL FIELD

The present invention generally relates to flash memory devices such as EEPROMs. More particularly, the present invention relates to flash memory devices improved word line performance, improved poly 2 performance and which do not exhibit silicide cracking.

BACKGROUND ART

Nonvolatile memory devices include flash EEPROMs (electrical erasable programmable read only memory devices). FIG. 1 represents the relevant portion of a typical flash memory cell 10. The memory cell 10 typically includes a source region 12, a drain region 14 and a channel region 16 in a substrate 18; and a stacked gate structure 20 overlying the channel region 16. The stacked gate 20 includes a thin gate dielectric layer 22 (commonly referred to as the tunnel oxide) formed on the surface of the substrate 18. The stacked gate 20 also includes a polysilicon floating gate 24 which overlies the tunnel oxide 22 and an interpoly dielectric layer 26 which overlies the floating gate 24. The interpoly dielectric layer 26 is often a multilayer insulator such as an oxide-nitride-oxide (ONO) layer having two oxide layers 26 a and 26 b sandwiching a nitride layer 26 c. A polysilicon control gate 28 overlies the interpoly dielectric layer 26 and a conductive layer 30, such as a tungsten silicide layer, overlies the polysilicon control gate 28. The conductive layer 30 may constitute, in part, a word line. The channel region 16 of the memory cell 10 conducts current between the source region 12 and the drain region 14 in accordance with an electric field developed in the channel region 16 by the stacked gate structure 20.

Generally speaking, a flash memory cell is programmed by inducing hot electron injection from a portion of the substrate, such as the channel section near the drain region, to the floating gate. Electron injection carries negative charge into the floating gate. The injection mechanism can be induced by grounding the source region and a bulk portion of the substrate and applying a relatively high positive voltage to the control electrode to create an electron attracting field and applying a positive voltage of moderate magnitude to the drain region in order to generate “hot” (high energy) electrons. After sufficient negative charge accumulates on the floating gate, the negative potential of the floating gate raises the threshold voltage (V_(th) of its field effect transistor (FET) and inhibits current flow through the channel region through a subsequent “read” mode. The magnitude of the read current is used to determine whether or not a flash memory cell is programmed. The act of discharging the floating gate of a flash memory cell is called the erase function. The erase function is typically carried out by a Fowler-Nordheim tunneling mechanism between the floating gate and the source region of the transistor (source erase or negative gate erase) or between the floating gate and the substrate (channel erase). A source erase operation is induced by applying a high positive voltage to the source region and a 0 V to the control gate and the substrate while floating the drain of the respective memory cell.

Referring still to FIG. 1, conventional source erase operations for the flash memory cell 10 operate in the following manner. The memory cell 10 is programmed by applying a relatively high voltage V_(G) (e.g., approximately 12 volts) to the control gate 28 and a moderately high voltage V_(D) (e.g., approximately 9 volts) to the drain region 14 in order to produce “hot” electrons in the channel region 16 near the drain region 14. The hot electrons accelerate across the tunnel oxide 22 and into the floating gate 24 and become trapped in the floating gate 24 since the floating gate 24 is surrounded by insulators (the interpoly dielectric 26 and the tunnel oxide 22). As a result of the trapped electrons, the threshold voltage of the memory cell 10 increases by about 3 to 5 volts. This change in the threshold voltage (and thereby the channel conductance) of the memory cell 10 created by the trapped electrons causes the cell to be programmed.

To read the flash memory cell 10, a predetermined voltage V_(G) that is greater than the threshold voltage of an unprogrammed cell, but less than the threshold voltage of a programmed cell, is applied to the control gate 28. If the memory cell 10 conducts, then the memory cell 10 has not been programmed (the cell 10 is therefore at a first logic state, e.g., a zero “0”). Likewise, if the memory cell 10 does not conduct, then the memory cell 10 has been programmed (the cell 10 is therefore at a second logic state, e.g., a one “1”). Consequently, it is possible to read each cell 10 to determine whether or not it has been programmed (and therefore identify its logic state).

In order to erase the flash memory cell 10, a relatively high voltage V_(s) (e.g., approximately 12 volts) is applied to the source region 12 and the control gate 28 is held at a ground potential (V_(G)=0), while the drain region 14 is allowed to float. Under these conditions, a strong electric field is developed across the tunnel oxide 22 between the floating gate 24 and the source region 12. The electrons that are trapped in the floating gate 24 flow toward and cluster at the portion of the floating gate 24 overlying the source region 22 and are extracted from the floating gate 24 and into the source region 12 by way of Fowler-Nordheim tunneling through the tunnel oxide 22. Consequently, as the electrons are removed from the floating gate 24, the memory cell 10 is erased.

In the manufacture of flash memory cells, there are a number of concerns when forming the second polysilicon layer and the conductive layer over the second polysilicon layer, especially when the first polysilicon layer (floating gate) and the ONO dielectric layer are patterned prior to formation of the second polysilicon layer. For example, the conductive layer should adequately adhere to the polysilicon control gate. The inability of the conductive layer to adequately adhere to the polysilicon control gate is typically due to delamination of the conductive layer. Delamination is sometimes caused or exasperated by peaks and valleys in the second polysilicon layer. Peaks and valleys in the second polysilicon layer are, in turn, attributable to the patterned first polysilicon layer and the patterned ONO dielectric layer.

The conductive layer should also effectively conduct an electrical current. However, current fabrication methods for flash memory devices result in deleterious microcracking within the tungsten silicide conductive layer. This is typically due to poor step coverage when forming a tungsten silicide conductive layer, which in turn is attributable to peaks and valleys in the second polysilicon layer. Microcracking leads often to undesirably increased word line resistance and poor polysilicon control gate performance.

Referring to FIG. 2, deleterious microcracking is illustrated. Specifically, microcracking 36 in tungsten silicide conductive layer 35 typically occurs near non-flat regions of the second polysilicon layer 34, such as over the field oxide region 31. The non-flat regions of the second polysilicon layer 34 often correspond to areas where the ONO dielectric layer 33 and the first polysilicon layer 32 are patterned or etched.

In view of the aforementioned concerns and problems, there is a need for flash memory cells of improved quality and more efficient methods of making such memory cells.

SUMMARY OF THE INVENTION

As a result of the present invention, a flash memory cell having improved reliability is obtainable by providing an improved tungsten silicide conductive layer. By forming the second polysilicon layer (poly 2) in accordance with the present invention, cracking of the tungsten silicide layer is minimized and/or eliminated. The present invention also makes it possible to decrease word line resistance and decrease poly 2 resistance. Accordingly, the present invention improves the electrical properties of the flash memory cell, and the tungsten silicide conductive layer in particular.

In one embodiment, the present invention relates to a method of forming a flash memory cell, involving the steps of forming a tunnel oxide on a substrate; forming a first polysilicon layer over the tunnel oxide; forming an insulating layer over the first polysilicon layer; forming a second polysilicon layer over the insulating layer by depositing an second polysilicon layer having a first thickness, and then using chemical mechanical polishing to form a second polysilicon layer having a second thickness, wherein the second thickness is at least about 25% less than the first thickness; forming a tungsten silicide layer over the second polysilicon layer by chemical vapor deposition using WF₆ and SiH₄; etching at least the first polysilicon layer, the second polysilicon layer, the insulating layer, and the tungsten silicide layer thereby defining at least one stacked gate structure; and forming a source region and a drain region in the substrate, thereby forming at least one memory cell.

In another embodiment, the present invention relates to a method of forming a control gate of polysilicon for a flash memory cell having a substantially flat top surface, involving forming a polysilicon layer having a thickness of at least about 2,000 Å, and chemical mechanical polishing to form a polysilicon layer having a thickness from about 1,000 Å to about 1,400 Å to provide the control gate.

In yet another embodiment, the present invention relates to a method of minimizing cracking of a tungsten silicide layer in a flash memory cell comprising a substrate, a tunnel oxide over the substrate, a floating gate over the tunnel oxide, an ONO multilayer dielectric over the floating gate, a control gate over the ONO insulating layer, and the tungsten silicide layer over the control gate, involving the steps of forming the control gate by depositing a polysilicon layer having a thickness of at least about 2,000 Å, implanting a polishing rate improver into the polysilicon layer, and chemical mechanical polishing the polysilicon layer to a thickness from about 1,000 Å to about 1,400 Å.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of relevant portions of a conventional memory cell in a flash memory cell.

FIG. 2 is a partial, cross-sectional view of relevant portions of a conventional memory cell in a flash memory cell.

FIGS. 3A to 3H are cross-sectional views illustrating a flash memory cell fabrication method according to one embodiment of the present invention.

DISCLOSURE OF INVENTION

By forming a second polysilicon layer having reduced peaks and valleys, it is possible to form a tungsten silicide layer having a lower stress value and less microcracks. The second polysilicon layer having a tungsten silicide layer with a relatively lower stress value and less microcracks thereover consequently has decreased resistance. While not wishing to be bound to any theory, it is believed that by forming a second polysilicon layer according to the present invention, it is consequently possible to prevent the occurrence of microcracks due to good step coverage of the subsequently formed tungsten silicide layer. By preventing microcracks in the tungsten silicide layer, word line resistance is decreased thus improving the performance of resultant flash memory devices. By decreasing poly 2 resistance, the performance of the resultant flash memory devices is improved.

According to the present invention, the second polysilicon layer (control gate) of a flash memory cell, typically formed over an ONO dielectric layer (an oxide/nitride/oxide multilayer dielectric layer), is formed using two step process. The flash memory cells according to the invention contain a silicon substrate, a tunnel oxide layer over the silicon substrate, a first polysilicon layer (floating gate) over the tunnel oxide layer, an ONO dielectric layer over the first polysilicon layer, the second polysilicon layer as described herein and a tungsten silicide layer. The process conditions involve forming the second polysilicon layer using a two step process to substantially planarize the second polysilicon layer and consequently minimize microcracking of the tungsten silicide layer formed over the second polysilicon layer.

The two step process of forming the second polysilicon layer involves initially forming a relatively thick doped or undoped polysilicon layer followed by using chemical mechanical polishing (CMP) techniques to reduce the initially thick polysilicon layer to a desired thickness. The resultant second polysilicon layer has a top surface that is substantially flat and parallel with the substrate surface. Consequently, the tungsten silicide layer formed over the second polysilicon layer also has a top surface that is a substantially flat and parallel with the substrate surface.

Specifically, the second polysilicon layer is made by initially forming a relatively thick doped or undoped polysilicon layer using any suitable technique. In one embodiment, a doped polysilicon layer is formed using an in situ doping method. For example, a phosphorus doped amorphous polysilicon layer may be deposited via CVD to form a doped polysilicon layer using SiH₄ and a mixture of PH₃ and a carrier gas such as helium. Carrier gases include one or more of the noble gases (He, Ne, Ar, Kr, and Xe) and nitrogen. In another embodiment, the initial polysilicon layer may be deposited by LPCVD optionally followed by ion implantation of a dopant such as phosphorus. The ion implantation may be conducted just after LPCVD, or after the CMP step described below.

In one embodiment, the thickness of the initially formed polysilicon layer is at least about 2,000 Å. In another embodiment, the thickness of the initially formed polysilicon layer is at least about 2,500 Å. In yet another embodiment, the thickness of the initially formed polysilicon layer is at least about 3,000 Å.

Optionally, the initially formed polysilicon layer is implanted with a polishing rate improver, which serves to improve the polishing rate of the CMP step. Polishing rate improvers include elements improve the polishing rate of CMP when implanted in doped or undoped polysilicon. Polishing rate improvers include at least one of silicon and germanium. The polishing rate improver(s) are implanted at a concentration suitable to enhance and/or facilitate CMP.

If employed, the polishing rate improver(s) are implanted substantially to the depth at which CMP is terminated. For example, if an initially formed polysilicon layer has a thickness of about 2,800 Å and about 50% of the thickness is removed by CMP, a polishing rate improver is implanted so that substantially all implanted species are at a depth of about 1,400 Å or less. In one embodiment, the polishing rate improver is implanted to a depth of at least about 1,000 Å in the initially formed polysilicon layer. In another embodiment, the polishing rate improver is implanted to a depth of at least about 1,300 Å in the initially formed polysilicon layer. In yet another embodiment, the polishing rate improver is implanted to a depth of at least about 1,600 Å in the initially formed polysilicon layer.

CMP techniques are employed to reduce the thickness and planarize the initially formed polysilicon layer. Any suitable CMP technique is employed. In one embodiment, CMP is employed to reduce the thickness of the initially formed polysilicon layer by at least about 25%. In another embodiment, CMP is employed to reduce the thickness of the initially formed polysilicon layer by at least about 50%. In yet another embodiment, CMP is employed to reduce the thickness of the initially formed polysilicon layer by at least about 60%.

In one embodiment, the polysilicon layer has a thickness of about 1,000 Å to about 1,500 Å after CMP. In another embodiment, the polysilicon layer has a thickness of about 1,100 Å to about 1,400 Å after CMP. In yet another embodiment, the polysilicon layer has a thickness of about 1,150 Å to about 1,300 Å after CMP.

The tungsten silicide layer is formed in any suitable manner, although using tungsten hexafluoride and monosilane with CVD techniques are preferred. Depositing the tungsten silicide is conducted for a time sufficient to form a tungsten silicide layer of desired thickness.

The thickness of the tungsten silicide layer is sufficient to provide a resultant tungsten silicide layer having a thickness from about 1,000 Å to about 1,800 Å, after optional anneal if performed. In one embodiment, the resultant tungsten silicide layer has a thickness from about 1,100 Å to about 1,700 Å. In another embodiment, the resultant tungsten silicide layer has a thickness from about 1,200 Å to about 1,600 Å.

The tungsten silicide deposition over the second polysilicon layer according to the present invention forms a tungsten silicide layer having a low or no occurrence of microcracking. As a result, the tungsten silicide layer made in accordance with the present invention has low resistance compared to tungsten silicide layers made over conventional second polysilicon layers.

After the tungsten silicide deposition, an optional anneal step is performed. Although not required, annealing the tungsten silicide layer can, in some instances, further contribute to the ability of the tungsten silicide layer to resist microcracking. In a preferred embodiment, a rapid thermal anneal (RTA) is performed. Annealing of the tungsten silicide layer is preferably conducted in an ambient atmosphere, such as a nitrogen (N₂) atmosphere. In one embodiment, annealing is conducted at a temperature from about 850° C. to about 950° C. In a preferred embodiment, annealing is conducted at a temperature from about 875° C. to about 925° C. In one embodiment, annealing is conducted for a time from about 10 seconds to about 30 seconds. In a preferred embodiment, annealing is conducted for a time from about 15 seconds to about 25 seconds. The annealing primarily serves to further decrease the occurrence of microcracking in the tungsten silicide layer.

Flash memory cells made in accordance with the present invention (containing a two step second polysilicon layer as a control gate) have high current densities, especially when compared with flash memory cells made with conventional second polysilicon layers. Since less charge trapping is associated with high current densities, flash memory cells made in accordance with the present invention have less charge trapping compared with flash memory cells made with conventional second polysilicon layers.

Referring to FIGS. 3A to 3H, the fabrication of a single flash memory cell is described. A plurality of flash memory cells can be formed on a semiconductor substrate, such as a silicon die, each with an N-type source region and N-type drain region formed within a P portion of the substrate and a P-type channel region interposed between the source and drain regions in accordance with the present invention. Although fabrication of one flash memory cell is described below, it will be understood by those skilled in the art that the methods described herein are applicable to mass production methods wherein two or more cells are formed.

Specifically referring to FIG. 3A, a P-type substrate 40 is provided. Thereafter, a thin tunnel oxide layer 42 is formed over the substrate 40 having a thickness of, for example, about 50 Å to about 150 Å using a thermal growth process in a dry oxidation furnace. For instance, the tunnel oxide layer 42 can be formed via dry oxidation at a temperature of about 1050° C., under an atmosphere of oxygen at about 1.33 l, HCl at about 70 cc and argon at about 12.6 l. Alternatively, the tunnel oxide can be formed from oxynitride.

Referring to FIG. 3B, a first polysilicon layer 44 is formed in any suitable manner over the tunnel oxide layer 42. For example, a phosphorus doped polysilicon layer is deposited via CVD to form a doped polysilicon layer 44 at 530° C., 400 mTorr, SiH₄ at 2000 sccm, and a mixture of 1% by weight PH₃ in helium at about 22 sccm. Doping lowers the resistivity of the polysilicon rendering it conductive.

A multi-layer interpoly dielectric 46 is then formed in any suitable manner over the surface of the substrate 40, as illustrated in FIG. 3C. This layer 46 is often called the interpoly dielectric since (as will be seen shortly) it is sandwiched between the phosphorus doped polysilicon layer (first polysilicon layer constituting the floating gate for a flash memory cell) and a second polysilicon layer which forms the control gate for the cell. The interpoly dielectric 46 is preferably a three layer region of oxide/nitride/oxide (the so called “ONO” layer) and typically has a total thickness of about 120 Å to about 400 Å. Generally speaking, the ONO layer 46 is formed by repetitive depositions of oxide, nitride and oxide to form a dielectric layer in which the nitride is sandwiched between a bottom oxide layer and top oxide layer.

Specifically referring to FIG. 3C, a first or bottom oxide layer 46 a is deposited using CVD techniques. For example, a bottom oxide layer 46 a is deposited at a temperature of about 750° C. under SiH₄ at 20 seem, N₂O at 1.2 l, and a carrier gas and a pressure of 600 mTorr via LPCVD oxide on the first polysilicon layer. The bottom oxide layer may have a suitable thickness, for example, from about 40 Å to about 60 Å, but typically the thickness is about 50 Å. A nitride layer 46 b is next deposited also using CVD techniques. For example, nitride is deposited at a temperature of about 760° C. under NH₃ at 600 sccm, SiH₂Cl₂ at 100 sccm and a pressure of 330 mTorr to form a nitride layer 46 b. The nitride layer 46 b may have a suitable thickness, for example, from about 60 Å to about 100 Å, preferably from about 70 Å to about 90 Å, but typically the thickness is about 80 Å.

The second or top oxide layer 46 c is formed in a similar manner as the first oxide layer or by wet oxidation at about 950° C. for about 40 minutes using oxygen (about 5 l) and hydrogen (about 9 l) to provide an oxide layer. The top oxide layer may have a thickness from about 20 Å to about 80 Å, but in this embodiment the thickness is about 40 Å. Although not shown, the ONO layer 46 and the first polysilicon layer 44 may be etched/patterned to form the floating gates of subsequently formed flash memory cells.

Referring to FIG. 3D, an initial second polysilicon layer is deposited in any suitable manner. Specifically, a phosphorus doped amorphous polysilicon layer is deposited via CVD to form a doped polysilicon layer 48′ at about 530° C., 400 mTorr, SiH₄ at 2,000 sccm, and a mixture of 1% by weight PH₃ in helium at about 75 sccm. Alternatively, the second polysilicon layer can be deposited by LPCVD followed by ion implantation of a dopant such as phosphorus. Doping lowers the resistivity of the polysilicon rendering it conductive. The thickness of the initial second polysilicon layer is about 3,000 Å. Silicon or germanium atoms are implanted to a depth of about 1,800 Å. Suitable CMP techniques are employed to reduce the thickness of the second polysilicon layer to about 1,200 Å. The top surface of the polysilicon layer 48 is substantially flat and parallel with the surface of the substrate.

Referring to FIG. 3E, a tungsten silicide layer 50 is deposited via LPCVD. The tungsten silicide layer 50 provides a lower resistance contact for improved flash memory cell performance. Specifically, the tungsten silicide is deposited at a temperature of about 360° C. and a pressure of about 200 mT. The gas flow includes SiH₄ and WF₆ and optionally a carrier gas. The gas flow contains about 500 sccm SiH₄ and about 50 scem WF₆. The gas flow may optionally further contain about 150 sccm to about 200 sccm of an inert gas, such as Ar. The thickness of the tungsten silicide layer is about 1,500 Å. After the tungsten silicide deposition, an optional RTA is performed in a nitrogen atmosphere. RTA is conducted at about 900° C. for about 20 seconds.

The second polysilicon layer according to the present invention leads to the formation of a tungsten silicide layer having no detectable microcracking. As a result, the tungsten silicide layer made in accordance with the present invention has low resistance compared to tungsten silicide layers made over conventional second polysilicon layers.

The remaining steps are generally well known in the art and may be varied. For instance, poly-cap layer 52 is deposited over the tungsten silicide layer 50. The poly-cap layer 52 is about 500 Å thick, and is formed via, for example, LPCVD. The poly-cap layer 52 can be used to prevent any potential peeling or cracking of the underlying tungsten silicide 50. A capping layer 54, for example, of SiON is deposited over the poly-cap layer 52. The capping silicon oxynitride layer 54 provides an anti-reflective coating at masking and also acts as a masking layer for subsequent etching.

Referring to FIGS. 3F, suitable lithography and etching procedures are used to remove various portions of the device. After the second polysilicon layer 48, the tungsten silicide layer 50, the poly-cap layer 52 and the capping layer 54 have been formed (a plurality of word lines for the memory cells can be defined in this manner) etching is performed to define one or more pre-stack structures. The etching may be achieved by depositing and defining a photoresist masking layer over the entire surface of the substrate using standard lithography procedures. This is generally termed the gate mask and gate etch. Subsequently, a number of successive etching steps, such as the gate etch and the self aligned etch, are performed to define one or more stack structures 56. This is generally termed the self aligned mask and self aligned etch.

The gate mask and gate etch are performed as follows. First, a resist (not shown) is applied, selectively exposed to radiation and developed whereby various portions removed (either the exposed or unexposed portions). Next, in one embodiment, the etching steps take place in a multi-chamber etch tool wherein a silicon oxynitride capping layer is selectively etched with a fluorinated chemistry such as CHF₃—O₂ in an oxide chamber. The exposed poly-cap layer and the tungsten silicide layer are then etched with SF₆/HBr (or alternatively, SF₆/Cl₂ or Cl₂—O₂) and the exposed second polysilicon layer is etched with HBr—O₂ in a poly chamber. Etching steps are preferably formed in an integrated process in which the wafers are not exposed to atmosphere when transferring the wafers from one chamber to another.

Once the second polysilicon layer 48, the tungsten silicide layer 50, the poly-cap layer 52 and the capping layer 54 have been removed, a self aligned etch (“SAE”) is performed to remove the ONO layer 46 and the phosphorus doped polysilicon layer (first polysilicon layer) 44 in the regions that are not covered by the pre-stack structure (constituted by the unremoved second polysilicon layer, tungsten silicide layer, poly-cap layer and capping layer) if these layers are not already etched. The SAE etch is a two step etch process in which the ONO layer 46 is first removed using, for example, a CF₄—O₂ RIE etch. The second phase of the SAE etch is the removal of the exposed first polysilicon layer 44 to thereby further define the floating gate structures for each respective word line. The polysilicon etch includes, for example, an HBr—O₂ or a HBr—Cl₂—O₂ RIE etch chemistry. The gate etch and SAE serve to define the stack structure 56.

The fabrication of the flash memory cells is then completed by forming the source and drain regions by, for example, ion implantation. During the formation of the source and drain regions, the stacked gate structure 56 serves as a self-aligning mechanism. Specifically referring to FIG. 3G, resist 62 is applied and selectively stripped followed by performing a first ion implantation using phosphorus (1×10¹⁴ ions/cm² at 60 KeV) to form an N-type source region 64 (double diffused implant). Referring to FIG. 3H, resist 62 is removed followed by performing a second ion implantation using arsenic (5×10¹⁴ ions/cm² at 40 KeV) to form deep N-type source region 66, shallow N-type source region 68 and N-type drain region 70 (modified drain diffusion). Annealing completes the formation of the source and drain regions. In the above manner, an easy method for forming flash memory cells is provided. Although a flash memory cell with a double-diffused source region is described, the present invention is also applicable to flash memory cells with a single-diffused source region.

During programming, the source regions 66 and 68 and the substrate 40 of the memory cell may be tied to a ground via a terminal (not shown), respectively, the drain region 70 is coupled to a relatively high voltage (for example, between about +5 V to about +9 V) via a terminal (not shown) and the control gate 48 is connected to a relatively high voltage level (for example, above about +10 V) via a terminal (not shown). Electrons are accelerated from the source regions 66 and 68 to the drain region 70 and so-called “hot-electrons” are generated near the drain region 70. Some of the hot electrons are injected through the relatively thin tunnel oxide layer 42 and become trapped in the floating gate 44 thereby providing the floating gate 44 with a negative potential.

During erasure, a high positive voltage (such as above about +12 V) is applied to the source regions 66 and 68 via a source terminal (not shown). A ground potential (V_(g) equals 0 V) is applied to the control gate 48 via the control terminal (not shown). A similar ground potential (V_(sub) equals 0 V) is applied to the substrate 40 via a substrate terminal (not shown). The voltage V_(D) of the drain region 70 is permitted to float. In this mode, electrons previously stored during programming in the floating gate 44 pass through tunnel oxide layer 42 by way of Fowler-Nordheim tunneling and travel into the source regions 66 and 68 as a result of the electric field established between the control gate 48 and the source regions 66 and 68 (V_(GS) equals about 12 V).

Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiments of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several embodiments, such feature may be combined with one or more other features of the other embodiments as may be desired and advantageous for any given or particular application. 

What is claimed is:
 1. A method of forming a flash memory cell, comprising: forming a tunnel oxide on a substrate; forming a first polysilicon layer over the tunnel oxide; forming an insulating layer over the first polysilicon layer; forming a second polysilicon layer over the insulating layer by depositing a second polysilicon layer having a first thickness, the first thickness being at least about 2,000 Å, implanting a polishing rate improver into the second polysilicon layer and then using chemical mechanical polishing to form a second polysilicon layer having a second thickness, wherein the second thickness is at least about 25% less than the first thickness; forming a tungsten silicide layer over the second polysilicon layer by chemical vapor deposition using WF₆ and SiH₄; etching at least the first polysilicon layer, the second polysilicon layer, the insulating layer, and the tungsten silicide layer thereby defining at least one stacked gate structure; and forming a source region and a drain region in the substrate, thereby forming at least one memory cell.
 2. The method of claim 1, wherein the second thickness is at least about 50% less than the first thickness.
 3. The method of claim 1, wherein the second polysilicon layer having a first thickness is deposited by chemical vapor deposition using SiH₄ and a mixture of PH₃ and a carrier gas.
 4. The method of claim 1, wherein the second polysilicon layer having a first thickness is deposited by low pressure chemical vapor deposition using SiH₄ followed by ion implantation of phosphorus.
 5. The method of claim 1, wherein the polishing rate improver is at least one of silicon and germanium.
 6. The method of claim 1, wherein the insulating layer comprises a first oxide layer over the first polysilicon layer, a nitride layer over the first oxide layer, and a second oxide layer over the nitride layer.
 7. A method of forming a control gate of polysilicon for a flash memory cell having a substantially flat top surface, comprising: forming a polysilicon layer having a thickness of at least about 2,000 Å, implanting a polishing rate improver into the polysilicon layer, and chemical mechanical polishing to form a polysilicon layer having a thickness from about 1,000 Å to about 1,400 Å to provide the control gate.
 8. The method of claim 7, wherein the polysilicon layer having the thickness of at least about 2,000 Å is doped with phosphorus.
 9. The method of claim 7, wherein the polysilicon layer having the thickness from about 1,000 Å to about 1,400 Å is doped with phosphorus.
 10. The method of claim 7, wherein the polysilicon layer having the thickness of at least about 2,000 Å has a thickness of at least about 2,500 Å.
 11. The method of claim 7, wherein the polysilicon layer having the thickness from about 1,000 Å to about 1,400 Å has a thickness from about 1,100 Å to about 1,300 Å.
 12. A method of minimizing cracking of a tungsten silicide layer in a flash memory cell comprising a substrate, a tunnel oxide over the substrate, a floating gate over the tunnel oxide, an ONO multilayer dielectric over the floating gate, a control gate over the ONO insulating layer, and the tungsten silicide layer over the control gate, comprising: forming the control gate by deposition a polysilicon layer having a thickness of at least about 2,000 Å, implanting a polishing rate improver into the polysilicon layer, and chemical mechanical polishing the polysilicon layer to a thickness from about 1,000 Å to about 1,400 Å.
 13. The method of claim 12, wherein at least about 50% of the thickness of the polysilicon layer having a thickness of at least about 2,000 Å is removed by chemical mechanical polishing.
 14. The method of claim 12, wherein the polysilicon layer having a thickness of at least about 2,000 Å has a thickness of at least about 3,000 Å, and at least about 60% of the thickness of the polysilicon layer having a thickness of at least about 3,000 Å is removed by chemical mechanical polishing.
 15. The method of claim 12, wherein the polishing rate improver is implanted to a depth of at least about 1,000 Å.
 16. The method of claim 15, wherein the polishing rate improver is at least one of silicon and germanium.
 17. The method of claim 12, wherein the tungsten silicide layer is made using tungsten hexafluoride and monosilane and chemical vapor deposition techniques to a thickness from about 1,000 Å to about 1,800 Å.
 18. The method of claim 12, wherein the control gate has a top surface that is substantially parallel with the substrate. 